Know Your Depths

We had an opportunity recently to check depths in the mooring field at Ayala Cove and the docks at Sam’s Anchor Cafe. The results were surprising! At a zero tide:

  • Depths on the west and southwest sides of the mooring field (the side closest to the land) are 5 feet.
  • The middle of the mooring field is a pretty consistent 4 feet
  • The moorings located on the northeast side (the inside of the cove) have only 3.5 feet

Santorini and Orion draw nearly 6 feet, which means if you approach the mooring field from the normal direction in less than a 2.5 foot tide, you are going to get stuck! Here is a practical example. Tides for Saturday September 21 and Sunday September 22, 2013 indicate:

  • High at 1323 6.1; Low at 1935 0.3; High at 0218 5.2; Low at 0737 1.7

When you arrive at your northeast mooring ball at 1400, you will have over 9 feet of water … no problem. By about 1930, your keel will settle over 2 feet deep in mud which probably won’t be a problem because you plan on spending the night there (hopefully that motor boat beside you tied up to 2 balls, because if he only grabbed 1 he is going to swing all night and you aren’t). Unfortunately, at 0730, when you plan to leave to sail over to San Francisco for breakfast, you are at another low, and back to a foot deep in mud … in other words … stuck fast. Using the rule of twelfths (see below) to figure tidal changes, it will be about 0930 or 1000 before you have enough water to leave the mooring!

Sam’s Anchor Café is another location to be careful.  At a zero tide, there is 4 feet at the end of the left dock, and 3.5 feet at the end of the right dock.  Half way up the docks, there is 3 feet on the left dock and 2.5 feet on the right dock.

Now for the tip …

First, know your draft, and how your depth sounder reads compared to actual water depths. The only way to be sure of that is with a lead line. A simple one can be made by tying a 10 or 12 oz. round lead fishing weight to a 25 or 30 foot light weight line. Mark the measurements on the line (mine has a mark every 6 feet, with the last 6 feet marked in 1 foot intervals). Lower it into the water beside the boat to get an actual measurement of depth. Compare that to the depth sounder on the boat. If the line indicates 16 feet of water, the depth sounder shows 10 feet, and you know you have a 6 foot draft, then the sounder is reading from the bottom of the keel, and you have 10 feet of water under it.

Second, know what the tides are doing! Not just the highs and lows, but what is happening in the middle. After all, rarely will you arrive or depart right on the extreme. A good general rule to follow is called the Rule of Twelfths, which basically says 1 twelfth of the tide change will take place in the first hour after the high or low. 2 twelfths of the change will happen during the second hour, 3 twelfths during the 3rd and 4th hours, 2 twelfths during the 5th hour, and the final 1 twelfth during the sixth hour. Sounds challenging, but is actually pretty simple. For example … using round figures, lets say the tidal change (low to high) is expected to be 4’. 4’ divided by 12 equals 4”. At the 0800 low tide, you have 2’. One hour later (0900) you should have about 2’ 4”. One hour after that (1000) you will have about 3’. At 1100 about 4’, at noon 5’, at 1300 5’8″, and at 1400 6′. It’s not exact, but it will get you pretty close.

When you tie up, check your depth (use the lead line if you don’t have a working depth sounder you trust). Compare the depths with what will happen to the tide during your stay. Get out before the water goes away! If I have a 5’ draft with 5.5’ of water and a falling tide, I’m going to find somewhere else to go.

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6 Responses to Know Your Depths

  1. Tony Johnson says:

    I love stuff like this. Absolutely great work in every way, Don. Luckily, my boat’s got a swing keel so I don’t care, and I’m too old to do all that arithmetic.

  2. Harry Chesley says:

    According to the charts, the middle of the mooring field at Ayala is 8’8″, more than twice what you measured. Actual measurement obviously trumps charts, but I’m amazed there’s so much difference.

    So your conclusion is that you cannot overnight on the moorings at Ayala with Santorini, true?

  3. TW-Matt says:

    You can if you get there and get the sets closest to shore or around the other edges – just not in the center of the mooring field. The swirly current in there must cause sediment to pile in the middle.

  4. Harry Chesley says:

    But even in the deepest area, he measured only 5′, so Santorini’s 6′ keel would need a lowest tide of no lower than 1′, 2′ for safety…

    I guess the moral is to always measure it when you moor and do the calculations for the overnight least depth. Don’t trust the charts alone.

  5. Steve Bonham says:

    Anyone do soundings of the docks at Ayala? Based on the soundings at the moorings, I’ll guess that the safest approach would be to hug the ferry landing on the East side and curve around the SE bend – then dock. This way you avoid the center area of the cove.

    • Don Gilzean says:

      Hi Steve
      Yes, I took a lead line over to the Ayala Cove docks. The depths at the end of each finger range from eight to ten feet at a zero tide. Depth wise, there are two concerns going into the dock. First, don’t get too close to land on the back side of the dock. Second, there is a “hump” that is six or seven feet deep about fifty feet out from the dock as you approach. Basically, its in the middle of where the ferry docks and the start of the mooring field.
      Don

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